With the growing acceptance of wind turbines as a commercially viable source of energy, and the continuous drive for reducing the cost of the produced energy, the size of turbines continues to increase. At the time of writing, all major turbine manufacturers have turbines in the 2-3 MW range, and most are developing larger models in the 3-7 MW range. Such models will typically have rotor blades which are in the region of 50 to 70 m or even larger, and weighing in the region of 5-7 tonnes or more.
The handling of these large blades during manufacture, transport to the turbine site, during installation, repair or replacement becomes increasingly problematic. Conventional handling techniques involve the use of slings which encircle the blades in the chordwise direction, and are connected to a crane lifting hook or eye. The use of such slings is potentially problematic in that these slings do not make a direct fixed connection to the blade, and are thereby susceptible to relative sliding of the blade within the sling. Moreover, if not carefully arranged, the slings can exert potentially damaging forces on parts of the blades, particularly the relatively delicate trailing edge. If the blade is provided with dynamically operative structures such as trailing edge flaps, these are particularly susceptible to damage. In addition, there is the potential for damage to the blade's structural integrity, in that the blade structure is designed in order to accommodate loading during normal use when in its operating position mounted at its root end on the rotor hub, and is not designed having particular regard to loads during lifting when constrained at or near the central region of the blade.
It has previously been proposed in Applicant's WO2005/071261 to provide the blade with mounting holes which penetrate the top and bottom blade shells, with bracket-like handling components arranged against the opposite blade surfaces and connected by bolts extending through the mounting holes. Such a structure enjoys several benefits of thereby providing a fixed lifting point, and allowing a firm grip to be provided on the blade.
The present invention seeks to provide, in a development of this structure, a blade structure and associated lifting assembly which overcomes the drawbacks discussed above, and is able to provide effective and safe lifting in particular of very large blades, and with good load distribution characteristics.